The present invention relates generally to network interfacing, and more particularly, to a device and method for I/Q modulation, frequency translation and upsampling.
The transmission of various types of digital data between computers continues to grow in importance. The predominant method of transmitting such digital data includes coding the digital data into a low frequency base data signal and modulating the base data signal onto a high frequency carrier signal. The high frequency carrier signal is then transmitted across a network cable medium, via RF signal, modulated illumination, or other network medium, to a remote network node.
At the remote computing station, the high frequency carrier signal must be received and demodulated to recover the original base data signal. In the absence of any distortion of the carrier signal across the network medium, the received carrier would be identical in phase, amplitude, and frequency to the transmitted carrier and could be demodulated using known mixing techniques to recover the base data signal. The base data signal could then be recovered into digital data using known sampling algorithms.
One problem with such networks is that the network topology tends to distort the high frequency carrier signal due to numerous branch connections and different lengths of such branches causing numerous reflections of the transmitted carrier. The high frequency carrier is further distorted by spurious noise caused by electrical devices operating in close proximity to the cable medium. Such problems are even more apparent in a network which uses home telephone wiring cables as the network cable medium because the numerous branches and connections are typically designed for transmission of plain old telephone system (POTS) signals in the 3-10 kilohertz frequency and are not designed for transmission of high frequency carrier signals on the order of 7 Megahertz. Further yet, the high frequency carrier signal is further distorted by turn-on transients due to on-hook and off-hook noise pulses of the POTS utilizing the network cables.
Such distortion of frequency, amplitude, and phase of the high frequency carrier signal degrades network performance and tends to impede the design of higher data rate networks. Known techniques for compensating for such distortion and improving the data rate of a network include complex modulation schemes.
Utilizing a complex modulation scheme such as quadrature amplitude modulation (QAM) data, both the amplitude and phase of the high frequency carrier are modulated to represent I and Q components of a base data signal. Referring to FIG. 1, a 4-QAM modulation constellation 10 is shown. In operation, each data symbol is represented by an I-value of +1 or xe2x88x921 and a Q-value of +1 or xe2x88x921 such that the data symbol can be represented by one of the four modulation states 12(a)-(d) in constellation 10. Each constellation state 12(a)-12(d) represents a unique combination of carrier amplitude and phase. For example, constellation state 12(a) represents a carrier amplitude of 14 and a carrier phase 16.
A complex modulation transmitter typically uses a look up table to generate an I-channel and a Q-channel baud rate data signal. An upsampler then inserts additional sample values of zero to increase the input sample frequency to the desired carrier frequency. A complex mixer then mixes each of the I-channel signal and the Q-channel signal by digital sine waves and digital cosine waves of the carrier frequency as appropriate to generate a modulated carrier signal. Narrow band digital filters are then used to remove harmonics and to assure that the transmitted signal has a strong signal to noise ratio within the desired band without excessive noise in the side bands.
A problem with such systems is that a carrier frequency on the order of 7 MHz is typically represented by digital values clocked at a frequency on the order of 32 MHz. As such, a digital signal processor (DSP) implementation of a QAM (or I/Q) transmitter can consume many gates or may not even be possible to implement in a high speed DSP without architectural innovation. What is needed is a device and method for I/Q modulation, upsampling, and digital filtering that does not suffer the disadvantages of known systems.
A first object of the present invention is to provide a device for modulating a carrier signal comprising: (a) a mapper generating a first data signal at a first data value frequency; (b) an upsampling device to increase the data value frequency of the first channel data signal to a second data value frequency; and (c) a pulse shaper including a finite impulse response filter operating on the first channel data signal to generate a filtered first channel data signal with characteristics that provide for reduced side band noise when mixed with a carrier frequency sine wave form signal. The device may further include a pre-scaler, positioned between the mapper and the upsampling device, operating to multiply the first data signal by a value corresponding to a selected baud rate to control signal strength of a carrier.
The mapper may further generate a second channel data signal at the first data value frequency and the device may further include: (a) a second upsampling device to increase the data value frequency of the second channel data signal to a second data value frequency; (b) a second pulse shaper including a finite impulse response filter operating on the second channel data signal to generate a filtered second channel data signal with characteristics that provide for reduced side band noise when mixed with a carrier frequency cosine wave form signal; and (c) a second pre-scaler, positioned between the mapper and the second upsampling device, operating to multiply the second data signal by a value corresponding to a selected baud rate. An adder may be included for adding the result of the first pulse shaper and the second pulse shaper to generate a complex modulated carrier signal.
The first channel data signal and the second channel data signal may be an I-channel data signal and a Q-channel data signal respectively and the output of the adder may be a quadrature amplitude modulated carrier signal.
A second objective of the present invention is to provide a method of modulating a carrier signal, the method comprising: (a) generating a first channel data signal at a first data value frequency; (b) increasing the data value frequency of the first channel data signal to a second data value frequency; and (c) filtering the first channel data signal to generate a filtered first channel data signal with characteristics that provide for reduced side band noise when mixed with a carrier frequency sine wave form signal. The method may further include: (d) scaling the first channel data signal with a value selected to correspond with a selected baud rate to control signal strength of a carrier; (e) generating a second channel data signal at the first data value frequency; (f) increasing the data value frequency of the second channel data signal to the second data value frequency; (g) filtering the second channel data signal to generate a filtered second channel data signal with characteristics that provide for reduced side band noise when mixed with a carrier frequency cosine wave form signal; and (h) scaling the second channel data signal with a value selected to correspond with a selected baud rate to control signal strength of a carrier.
The method may further yet include adding the filtered first channel data signal with the filtered second channel data signal to generate a complex modulated carrier signal.
The first channel data signal and the second channel data signal may be an I-channel data signal and Q-channel data signal respectively and the complex modulated carrier signal may be a quadrature amplitude modulated carrier signal.
A third object of the present invention is to provide a device for modulating a carrier signal comprising: (a) a mapper generating a first channel data signal and a second channel data signal, both at a first data value frequency; (b) a complex mixer including:
(i) a first multiplier and a second multiplier each multiplying the first channel data signal by a sine wave form and a cosine wave form respectively;
(ii) a third and fourth multiplier each multiplying the second channel data signal by the sine waveform and a cosine wave form respectively;
(iii) a first channel summer adding the result of the second multiplier to the result of the third multiplier multiplied by negative one; and
(iv) a second channel summer adding the result of the first multiplier and the result of the fourth multiplier, the base rate sine waveform and the base rate cosine waveform having a frequency of one fourth the first sampling frequency;
(c) an upsampling device to increase the data value frequency of the result of the first channel summer and the result of the second channel summer; and (d) a finite impulse response filter operating on the first channel data signal and the second channel data signal respectively to generate a filtered first channel data signal and a filtered second channel data signal, both with characteristics that provide for reduced side band noise when the filtered second channel data signal is subtracted from the filtered first channel data signal.
The device may further include a third summer for subtracting the filtered second channel data signal from the filtered first channel data signal to generate a complex modulated carrier signal.
The result of the first channel summer may be an I-channel signal, the result of the second channel summer may be a Q-channel signal, and the output of the third summer may be a quadrature amplitude modulated carrier signal.
A fourth object of the present invention is to provide a method of modulating a carrier signal, the method comprising: (a) generating a first channel data signal and a second channel data signal, both at a first data value frequency; (b) performing complex mixing to generate:
(i) a first channel complex mixed signal resulting from subtracting the result of mixing the second channel data signal with a sine waveform from the result of mixing the first channel data signal with a cosine waveform; and
(ii) a second channel complex mixed signal resulting from adding the result of mixing the second channel data signal with a cosine waveform and the result of mixing the first channel data signal with a sine waveform, the sine waveform and the cosine waveform having a frequency of one fourth the first data value frequency;
(c) upsampling each of the first channel complex mixed signal and the second channel complex mixed signal to increase the data value frequency of; and (d) filtering each of the first channel complex mixed signal and the second channel complex mixed signal to generate each of a first channel filtered complex mixed signal and a second channel filtered complex mixed signal respectively, both with characteristics that provide for reduced side band noise when the second channel filtered complex mixed signal is subtracted from the first channel filtered complex mixed signal.
The method may further include subtracting the second channel filtered complex mixed signal from the first channel filtered complex mixed signal to generate a complex modulated carrier signal.
The first channel data signal and the second channel data signal may be an I-channel data signal and a Q-channel data signal respectively and the complex modulated carrier signal may be a quadrature amplitude modulated carrier signal.